PanelView: Use geometry() instead of geometryByDistance() for computing mask

GeometryByDistance() returns the place where the panel should be at
a given moment; however it might not have moved there yet, thus
it's better to use the actual screen geometry. If there's a mismatch
between the two, e.g. when moving the panel around, then the
mapFromGlobal() function would place the mask outside the screen area.

BUG: 478923
FIXED-IN: 6.0


(cherry picked from commit 1e6a908f)